U.S. patent number 5,793,371 [Application Number 08/511,294] was granted by the patent office on 1998-08-11 for method and apparatus for geometric compression of three-dimensional graphics data.
This patent grant is currently assigned to Sun Microsystems, Inc.. Invention is credited to Michael F. Deering.
United States Patent |
5,793,371 |
Deering |
August 11, 1998 |
Method and apparatus for geometric compression of three-dimensional
graphics data
Abstract
In a compression system, three-dimensional geometry is first
represented as a generalized triangle mesh, a data structure that
allows each instance of a vertex in a linear stream to specify an
average of two triangles. Individual positions, colors, and normals
are quantized, preferably quantizing normals using a novel
translation to non-rectilinear representation. A variable length
compression is applied to individual positions, colors, and
normals. The quantized values are then delta-compression encoded
between neighbors, followed by a modified Huffman compression for
positions and colors. A table-based approach is used for normals.
Decompression reverses this process. The decompressed stream of
triangle data may then be passed to a traditional rendering
pipeline, where it is processed in full floating point
accuracy.
Inventors: |
Deering; Michael F. (Los Altos,
CA) |
Assignee: |
Sun Microsystems, Inc. (Palo
Alto, CA)
|
Family
ID: |
24034288 |
Appl.
No.: |
08/511,294 |
Filed: |
August 4, 1995 |
Current U.S.
Class: |
345/418; 345/420;
382/232; 382/242; 700/253; 708/203 |
Current CPC
Class: |
G06T
9/001 (20130101) |
Current International
Class: |
G06T
9/00 (20060101); G06T 015/00 () |
Field of
Search: |
;345/202,418-9,430-1,501,503,420,428,429 ;395/888 ;382/232,242
;364/715.02 |
Other References
J W. Durkin and J. F. Hughes, Nonpolygonal Isosurface Rendering for
Large Volume Datasets, 1070-2385/94 1994 IEEE. .
J. F. Danskin, Ph.D. Compressing the X Graphics Protocol, Nov.
1994, Dissertiation, Princeton University, Dept. of Computer
Science, Princeton, New Jersey..
|
Primary Examiner: Herndon; Heather R.
Assistant Examiner: Buchel; Rudolph
Attorney, Agent or Firm: Conley, Rose & Tayon Hood;
Jeffrey C.
Claims
What is claimed is:
1. A method for compression in which a three-dimensional object
whose surface defines surface characteristics including at least
one characteristic selected from the group consisting of (i)
position, (ii) normals, (iii) colors, (iv) texture map coordinates,
(v) material surface properties of said object, said object being
representable as three-dimensional triangular data defining a
plurality of triangles having vertices, the method comprising:
(a) converting said triangular data into linear strip format;
(b) quantizing individual ones of said surface characteristics;
(c) applying variable length compression to a difference between
sequential ones said surface characteristics; and
(d) providing a data stream representing said three-dimensional
object as a collection of data generated in step (b) and step
(c).
2. The method of claim 1, wherein at step (a), said format has at
least one format characteristic selected from the group consisting
of (i) said format defines a generalized triangular data mesh, and
(ii) said format causes each linear strip vertex on average to
specify a number of triangles ranging from 1/3 to 2.
3. The method of claim 1, wherein said object has at least one
characteristic selected from the group consisting of (i)
characteristics of position of said object are represented as
rectilinear x, y, z coordinate, (ii) characteristics of color of
said object are represented as consisting of at least red, green,
blue, and alpha components, said color being selected from the
group consisting of at least (a) emissive color, (b) ambient color,
(c) diffuse color, and (d) specular color, and (iii)
characteristics of normals of said object are represented by
rectilinear Nx, Ny, and Nz vector coefficients.
4. The method of claim 1, wherein each of said normals is
represented by a single index into a table of predefined normalized
normals.
5. The method of claim 4, wherein a chosen one, two or three bits
of index representation comprise rectilinear representation sign
bits of a normal to be represented.
6. The method of claim 4, wherein said index includes at least one
bit specifying possible foldings of an absolute magnitude of a
rectilinear representation of the normal to be represented by at
least one folding plane x=y, x=z, y=z such that a folded normal is
ensured to be on a known side of said folding plane.
7. The method of claim 4, wherein said index includes bits
representing longitude information and latitude information of said
normal.
8. The method of claim 4, wherein said index is defined to include
three-bits of octant information, three-bits of sextant
information, and two additional fields of information about
longitude and latitude of the folded normal.
9. The method of claim 1, wherein step (c) is carried out at least
in part using a procedure selected from the group consisting of (i)
Huffman compression procedure, (ii) a variant of a Huffman
compression procedure, (iii) arithmetic coding, and (iv) a variant
of arithmetic coding.
10. The method of claim 4, wherein said index includes bits
representing longitude information and latitude information of said
normal, and further includes three-bits of octant information,
three-bits of sextant information, and two additional fields of
information about longitude and latitude of the folded normal,
wherein at step (c), said difference between normals may be
represented solely by differences between their longitude and
latitude information of said normals if other components of said
representation are otherwise identical.
11. A system for compressing data representing a three-dimensional
object whose surface defines surface characteristics including at
least one characteristic selected from the group consisting of (i)
position, (ii) normals, (iii) colors, (iv) texture map coordinates,
(v) material surface properties of said object, said object being
representable as three-dimensional triangular data defining a
plurality of triangles having vertices, the system comprising:
a computer system including a central processing unit (CPU) and
memory coupled to said CPU;
a first unit programmed to convert said triangular data into linear
strip format;
a quantizing unit that quantizes individual ones of said surface
characteristics;
a compression unit that applies variable length compression to a
difference between sequential ones said surface characteristics;
and
a data stream generator that provides a data stream representing
said three-dimensional object as a collection of data generated by
said quantizing unit and by said compression unit.
12. The system of claim 11, wherein said format has at least one
format characteristic selected from the group consisting of (i)
said format defines a generalized triangular data mesh, and (ii)
said format causes each linear strip vertex on average to specify a
number of triangles ranging from 1/3 to 2.
13. The system of claim 11, wherein characteristics of said object
have at least one characteristic selected from the group consisting
of (i) characteristics of position of said object are represented
as rectilinear x, y, z coordinates, (ii) characteristics of color
of said object are represented as consisting of at least red,
green, blue, and alpha components, said color being selected from
the group consisting of at least (i) emissive color, (ii) ambient
color, (iii) diffuse color, and (iv) specular color, and (iii)
characteristics of normals of said object are represented by
rectilinear Nx, Ny, and Nz vector coefficients.
14. The system of claim 11, wherein each of said normals is
represented by a single index into a table of predefined normalized
normals.
15. The system of claim 14, wherein a chosen one, two or three bits
of index representation comprise rectilinear representation sign
bits of a normal to be represented.
16. The system of claim 14, wherein said index includes at least
one bit specifying possible foldings of an absolute magnitude of a
rectilinear representation of the normal to be represented by at
least one folding plane x=y, x=z, y=z such that a folded normal is
ensured to be on a known side of said folding plane.
17. The system of claim 14, wherein said index includes bits
representing longitude information and latitude information of said
normal.
18. The system of claim 14, wherein said index is defined to
include three-bits of octant information, three-bits of sextant
information, and two additional fields of information about
longitude and latitude of the folded normal.
19. The system of claim 14, wherein said compression unit uses, at
least in part, a procedure selected from the group consisting of
(i) Huffman compression procedure, (ii) a variant of a Huffman
compression procedure, (iii) arithmetic coding, and (iv) a variant
of arithmetic coding.
20. The system of claim 14, wherein said index includes bits
representing longitude information and latitude information of said
normal, and further includes three-bits of octant information,
three-bits of sextant information, and two additional fields of
information about longitude and latitude of the folded normal,
wherein at step (c), said difference between normals may be
represented solely by differences between their longitude and
latitude information of said normals if other components of said
representation are otherwise identical.
Description
FIELD OF THE INVENTION
The present invention relates generally to compressing
three-dimensional graphics data, and more particularly to methods
and apparatuses that provide lossy high compression ratios for
three-dimensional geometry compression.
BACKGROUND OF THE INVENTION
Modern three-dimensional computer graphics use geometry extensively
to describe three-dimensional objects, using a variety of graphical
representation techniques. Computer graphics find wide use in
applications ranging from computer assisted design ("CAD") programs
to virtual reality video games. Complex smooth surfaces in of
objects can be succinctly represented by high level abstractions
such as trimmed non-uniform rational splines ("NURBs"), and often
detailed surface geometry can be rendered using texture maps. But
adding more realism requires raw geometry, usually in the form of
triangles. Position, color, and normal components of these
triangles are typically represented as floating point numbers, and
describing an isolated triangle can require upwards of 100 bytes of
storage space.
Understandably, substantial space is necessary for
three-dimensional computer graphics objects to be stored, e.g., on
a computer hard disk or compact disk read-only memory ("CD-ROM").
Similarly, considerable time in necessary for such objects to be
transmitted, e.g., over a network, or from disk to main memory.
Geometry compression is a general space-time trade-off, and offers
advantages at every level of a memory/interconnect hierarchy. A
similar systems problem exists for storage and transmission of
two-dimensional pixel images. A variety of lossy and lossless
compression and decompression techniques have been developed for
two-dimensional pixel images, with resultant decrease in storage
space and transmission time. Unfortunately, the prior art does not
include compression/decompression techniques appropriate for
three-dimensional geometry, beyond polygon reduction techniques.
However, the Ph.D. thesis entitled Compressing the X Graphics
Protocol by John Danskin, Princeton University, 1994 describes
compression for two-dimensional geometry.
Suitable compression can greatly increase the amount of geometry
that can be cached, or stored, in the fast main memory of a
computer system. In distributed networked applications, compression
can help make shared virtual reality ("VR") display environments
feasible, by greatly reducing transmission time.
Most major machine computer aided design ("MCAD") software
packages, and many animation modeling packages use constructive
solid geometry ("CSG") and free-form NURBS to construct and
represent geometry. Using such techniques, regions of smooth
surfaces are represented to a high level with resulting trimmed
polynomial surfaces. For hardware rendering, these surfaces
typically are pre-tessellated in triangles using software before
transmission to rendering hardware. Such software pre-tessellation
is done even on hardware that supports some form of hardware NURBS
rendering.
However, many advantages associated with NURBS geometric
representation are for tasks other than real-time rendering. These
non-rendering tasks include representation for machining,
interchange, and physical analysis such as simulation of turbulence
flow. Accurately representing trimming curves for NURBS is very
data intensive, and as a compression technique, trimmed NURBS can
not be much more compact than pre-tessellated triangles, at least
at typical rendering tessellation densities. Finally, not all
objects are compactly represented by NURBS. Although many
mechanical objects such as automobile hoods and jet turbine blades
have large, smooth areas where NURBS representations can be
advantageous, many objects do not have such areas and do not lend
themselves to such representation. Thus, while NURBS will have many
applications in modelling objects, compressed triangles will be far
more compact for many classes of application objects.
Photo-realistic batch rendering has long made extensive use of
texture map techniques to compactly represent fine geometric
detail. Such techniques can include color texture maps, normal bump
maps, and displacement maps. Texture mapping works quite well for
large objects in the far background, e.g., clouds in the sky,
buildings in the distance. At closer distances, textures work best
for three-dimensional objects that are mostly flat, e.g.,
billboards, paintings, carpets, marble walls, and the like. More
recently, rendering hardware has begun to support texture mapping,
and real-time rendering engines can also apply these
techniques.
However, texture mapping results in a noticeable loss of quality
for nearby objects that are not flat. One partial solution is the
"signboard", in which a textured polygon always swivels to face the
observer. But when viewed in stereo, especially head-tracked VR
stereo, nearby textures are plainly perceived as flat. In these
instances, even a lower detail but fully three-dimensional
polygonal representation of a nearby object would be much more
realistic.
Polyhedral representation of geometry has long been supported in
the field of three-dimensional raster computer graphics. In such
representation, arbitrary geometry is expressed and specified
typically by a list of vertices, edges, and faces. As noted by J.
Foley, et al. in Computer Graphics: Principles and Practice, 2nd
ed., Addison-Wesley, 1990, such representations as winged-edge data
structures were designed as much to support editing of the geometry
as display. Vestiges of these representations survive today as
interchange formats, e.g., Wavefront OBJ. While theoretically
compact, some compaction is sacrificed for readability by using
ASCII data representation in interchange files. Unfortunately, few
if any of these formats can be directly passed as drawing
instructions to rendering hardware.
Another historical vestige in such formats is the support of
N-sided polygons, a general primitive form that early rendering
hardware could accept. However, present day faster rendering
hardware mandates that all polygon geometry be reduced to triangles
before being submitted to hardware. Polygons with more than three
sides cannot in general be guaranteed to be either planar or
convex. If quadrilaterals are accepted as rendering primitives, it
is to be accepted that they will be arbitrarily split into a pair
of triangles before rendering.
Modern graphics languages typically specify binary formats for the
representation of collections of three-dimensional triangles,
usually as arrays of vertex data structures. Thus, PHIGS PLUS, PEX,
XGL, and proposed extensions to OpenGL are of this format form, and
will define the storage space taken by executable geometry.
It is known in the art to isolate or chain triangles in "zigzag" or
"star" strips. For example, Iris-GL, XGL, and PEX 5.2 define a form
of generalized triangle strip that can switch from a zigzag to
star-like vertex chaining on a vertex-by-vertex basis, but at the
expense of an extra header word per vertex in XGL and PEX. A
restart code allows multiple disconnected strips of triangles to be
specified within one array of vertices.
In these languages, all vertex components (positions, colors,
normals) may be specified by 32-bit single precision IEEE floating
point numbers, or 64-bit double precision numbers. The XGL, IrisGL,
and OpenGL formats also provide some 32-bit integer support. The
IrisGL and OpenGL formats support vertex position component inputs
as 16-bit integers, and normals and colors can be any of these as
well as 8-bit components. In practice, positions, colors, and
normals can be quantized to significantly fewer than 32 bits
(single precision IEEE floating point) with little loss in visual
quality. Such bit-shaving may be utilized in commercial
three-dimensional graphics hardware, providing there is appropriate
numerical analysis support.
In summation, there is a need for graphics compression that can
compress three-dimensional triangles, and whose format may be
directly passed as drawing instructions to rendering hardware.
Preferably such compression should be readily implementable using
real-time hardware, and should permit decompression using software
or hardware.
The present invention discloses such compression.
SUMMARY OF THE PRESENT INVENTION
According to the present invention, geometry is first represented
as a generalized triangle mesh, which structure allows each
instance of a vertex in a linear stream preferably to specify an
average of between 1/3 triangle and 2 triangles. Individual
positions, colors, and normals are quantized, with a variable
length compression being applied to individual positions, colors,
and normals. Quantized values are delta-compression encoded between
neighbors to provide vertex traversal orders, and mesh buffer
references are created. Histograms of delta-positions,
delta-normals and delta-colors are created, after which variable
length Huffman tag codes, as well as delta-positions, delta-normals
and delta-colors are created. The compressed output binary stream
includes the output Huffman table initializations, ordered vertex
traversals, output tags, and the delta-positions, delta-normals,
and delta-colors.
Decompression reverses this process. The decompressed stream of
triangle data may then be passed to a traditional rendering
pipeline, where it can be processed in full floating point
accuracy, and thereafter displayed or otherwise used.
Other features and advantages of the invention will appear from the
following description in which the preferred embodiments have been
set forth in detail, in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a generalized network system over which
three-dimensional graphics compressed according to the present
invention may be transmitted, and decompressed for user
viewing;
FIG. 2 depicts a generalized triangular mesh data structure, and
generalized mesh buffer representation of surface geometry,
according to the present invention;
FIG. 3 depicts six-way sign-bit and eight-way octant symmetry in a
unit sphere, used to provide forty-eight way reduction in table
look-up size, according to the present invention;
FIG. 4A depicts a vertex command in a geometry compression
instruction set, according to the present invention;
FIG. 4B depicts a normal command in a geometry compression
instruction set, according to the present invention;
FIG. 4C depicts a color command in a geometry compression
instruction set, according to the present invention;
FIG. 4D depicts a mesh buffer reference command in a geometry
compression instruction set, according to the present
invention;
FIG. 4E depicts a set state instruction in a geometry compression
instruction set, according to the present invention;
FIG. 4F depicts a set table command instruction in a geometry
compression instruction set, according to the present
invention;
FIG. 4G depicts a pass through command instruction in a geometry
compression instruction set, according to the present
invention;
FIG. 4H depicts a variable length no-op command instruction in a
geometry compression instruction set, according to the present
invention;
FIG. 4I depicts tag and .DELTA.-position data structure, according
to the present invention;
FIGS. 4J-1 and 4J-2 depict alternative tag and .DELTA.-normal data
structure, according to the present invention;
FIG. 4K depicts tag and .DELTA.-color data structure, according to
the present invention;
FIG. 5 is a flowchart of method steps in a geometry compression
algorithm, according to the present invention;
FIG. 6 is a block diagram of decompressor hardware, suitable for
use with the present invention;
FIGS. 7A-7L depict objects compressed under different conditions
with the present invention;
FIG. 8 is a detailed overall block diagram of a decompressor unit
suitable for decompressing data compressed according to the present
invention;
FIG. 9 is a detailed block diagram of the input block shown in FIG.
8;
FIG. 10 is a detailed block diagram of the barrel shifter unit
shown in FIG. 8;
FIG. 11 is a detailed block diagram of the position/color processor
unit shown in FIG. 8;
FIG. 12A is a detailed block diagram of the normal processor unit
shown in FIG. 8;
FIG. 12B is a detailed block diagram showing the decoder, fold, and
ROM look-up components associated with the normal processor unit of
FIG. 12A;
FIG. 13 is a block diagram showing interfaces to a mesh buffer, as
shown in FIG. 11 and/or FIG. 12A;
FIG. 14A depicts interfaces to Huffman tables;
FIG. 14B depicts a preferred format for entry of the Huffman table
data;
FIG. 15A depicts a vertex instruction;
FIG. 15B depicts vertex component data formats;
FIG. 15C depicts the format for the set normal instruction;
FIG. 15D depicts a set color instruction;
FIG. 15E depicts a mesh buffer reference instruction;
FIG. 15F depicts a set state instruction;
FIG. 15G depicts a set table instruction;
FIG. 15H depicts a passthrough instruction;
FIG. 15I depicts a variable-length NOP instruction; and
FIG. 15J depicts a skip 8 instruction.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A graphics compressor according to the present invention may be
used to reduce the space needed to store three-dimensional graphics
object, e.g., on a CD-ROM or the like, as well as to reduce the
time needed to transmit a compressed three-dimensional graphics
object, for example over a network. Before describing
three-dimensional graphics compression per se, the overall
environment in which the present invention may be practiced will be
described with respect to FIG. 1.
FIG. 1 depicts a generalized network with which three-dimensional
compression according to the present invention may advantageously
be used, to decrease storage space and to decrease time to transmit
compress three-dimensional graphics objects. Of course,
three-dimensional graphics compression according to the present
invention may be used in other environments as well, e.g., to
reduce requirements to store three-dimensional graphics on CD-ROMs,
to compress data in real-time, for example, in an interactive
television environment.
As shown in FIG. 1, a source of three-dimensional graphics data 10
may be coupled to a server or encoder system 20 whose processed and
compressed output is coupled over one or more networks 30 to one or
more target clients or decoder systems 40. The network may be
homogeneous, heterogeneous, or point-to-point.
Server 20 includes a central processing unit 50 that includes a
central processor unit per se ("CPU") 60 with associated main
memory 70, a mesh buffer 80, a memory portion 90 that preferably
contains an algorithm used to implement compression according to
the present invention, and a region of read-only-memory ("ROM")
100. ATTACHMENT 1 is a copy of a code listing for a preferred
embodiment of a compression algorithm, according to the present
invention. Alternatively, compression according to the present
invention may be carried out in hardware as opposed to
software.
Server 20 also includes a three-dimensional graphics compression
unit 60, whose compressed output data is arranged by a disk layout
unit 70 for storage onto storage disk unit 80, which may include
one or more CD-ROMs. The server communicates over the network(s) 30
via network interface unit 110. Those skilled in the art will
appreciate that server 20 may include a mechanism for arbitrating
between a plurality of client-decoder requests for compressed
data.
It is to be understood that the compressed three-dimensional
graphics data on video disk or CD-ROM 80 need not be transmitted
over a network. Disk or CD-ROM 80 may, for example, be mailed to a
user wishing to access the compressed three-dimensional graphics
information stored thereon. However, if transmitted, e.g., over a
network, transmission time will be advantageously reduced because
the compression substantially reduces the bit-size of the file to
be transmitted. Lossy compression of three-dimensional geometric
data according to the present invention can produce ratios of
six:one to ten:one, with little loss in displayed object quality.
Further, such compression can be included at relatively low cost
into real-time three-dimensional rendering hardware, or can instead
be implemented using purely software techniques.
In a network environment, at the receiving end, decoder systems(s)
40 include a central processing system 150 that includes a CPU 160,
memory 170, a portion of which 180 may include decompression
software, and ROM 190. Three-dimensional graphics that have been
compressed with the present invention may advantageously be
decompressed using software, hardware, or a combination of
each.
Decoder 40 further includes a network interface unit 120, a unit
130 that decompresses three-dimensional graphics data, and whose
output is coupled to a three-dimensional graphics rendering unit
140. The thus-decompressed three-dimensional graphics image(s) may
then be coupled to a viewer 200, or to another system requiring the
decompressed graphics. Of course, unit 40 may be a standalone unit,
into which three-dimensional graphics data, precompressed according
to the present invention, may be coupled for decompression. Unit 40
may, for example, comprise a computer or workstation.
Applicant's patent application Ser. No. 08/511,826 filed Aug. 4,
1995, entitled METHOD AND APPARATUS FOR DECOMPRESSION OF COMPRESSED
GEOMETRIC THREE-DIMENSIONAL GRAPHICS DATA, assigned to the assignee
here, discloses a preferred method and system for decompressing
data that has been compressed according to the present invention.
Attached hereto as ATTACHMENT 2 is a code listing of a
decompression algorithm with which such decompression may
preferably be implemented.
The operation of three-dimensional graphics compression unit 60
will now be described. In the present invention, the first stage of
geometry compression converts triangle data into an efficient
linear strip form, namely a generalized triangle mesh. For a given
fixed capacity of storage medium 80, a triangle mesh data structure
is a near-optimal representation of triangle data. In the preferred
embodiment, three-dimensional graphics object may be represented as
three-dimensional triangular data, whose format after conversion
causes each linear strip vertex, on average, to specify from about
1/3 triangles to about 2 triangles.
Further, a generalized triangle strip structure permits compact
representation of geometry while maintaining a linear data
structure. Stated differently, the compressed geometry can be
extracted by a single monotonic scan over the vertex array data
structure. This feature is advantageous for pipelined hardware
implementations.
FIG. 2 depicts a generalized triangle mesh data structure, and
generalized mesh buffer representation of surface geometry. Such a
mesh data structure may be used in three-dimensional geometry
compression, although by confining itself to linear strips, a
generalized triangle strip format wastes a potential factor of two
in space.
The geometry shown in FIG. 2, for example, can be represented by
one triangle strip, but many interior vertices will appear twice in
the strip.
In FIG. 2, a generalized triangle strip may be defined as follows,
where the R denotes restart, O denotes replace oldest, M denotes
replace middle, and a trailing letter p denotes push into mesh
buffer. The number following a capital letter is a vertex number,
and a negative number is the mesh buffer reference, in which -1
denotes the most recent pushed vertex.
R6, 01, 07, 02, 03, M4, M8, 05, 09, 010, M11 M17, M16, M9, 015, 08,
07, M14, 013, M6, 012, M18, M19, M20, M14, 021, 015, 022, 016, 023,
017, 024, M30, M29, M28, M22, 021, M20, M27, 026, M19, 025, 018
Using the same nomenclature, a generalized triangle mesh may be
defined as follows:
R6p, 01, 07p, 02, 03, M4, M8p, 05, 09p, 010, Mil, M17p, M16p, M-3,
015p, 0-5, 06, M14p, 013p, M9, 012, M18p, M19p, M20p, M-5, 021p,
0-7, 022p, 0-9, 023, 0-10, 0-7, M30, M29, M28, M-1, 0-2, M-3, M27,
026, M-4, 025, 0-5
It is to be noted that a vertex reference advantageously can be
considerably more compact (e.g., be represented by fewer bits) than
a full vertex specification.
Geometry compression according to the present invention explicitly
pushes old vertices (e.g., vertices with a trailing letter "p"
above) into a queue associated with mesh buffer memory 80 (see FIG.
1). These old vertices will later be explicitly referenced when the
old vertex is desired again. This approach provides a fine control
that supports irregular meshes of nearly any shape. In practice,
buffer memory 80 has finite length, and in the preferred embodiment
a maximum fixed queue length of 16 is used, which requires a 4-bit
index. As used herein, the term "mesh buffer" shall refer to this
queue, and the expression "generalized triangle mesh" will refer to
a combination of generalized triangle strips and mesh buffer
references.
The fixed size of mesh buffer 80 requires all
tessellators/re-strippers for compressed geometry to break-up any
runs longer than sixteen unique references. However, as geometry
compression typically will not be programmed directly at the user
level but rather by sophisticated tessellators/reformatters, this
restriction is not onerous. Sixteen old vertices can in fact permit
avoiding re-specification of up to about 94% of the redundant
geometry.
FIG. 2 also is an example of a general mesh buffer representation
of surface geometry. Geometry compression language supports the
four vertex replacement codes of generalized triangle strips,
namely: replace oldest, replace middle, restart clockwise, and
restart counterclockwise. Further, the language adds an additional
bit in each vertex header to indicate whether or not this vertex
should be pushed into the mesh buffer. In the preferred embodiment,
the mesh buffer reference command has a 4-bit field to indicate
which old vertex should be re-referenced, along with the 2-bit
vertex replacement code. Mesh buffer reference commands do not
contain a mesh buffer push bit; old vertices can only be recycled
once.
In practice, geometry rarely is comprised purely of positional
data. In general, a normal, and/or color, and/or texture map
coordinate are also specified per vertex. Accordingly, in the
preferred embodiment, entries into mesh buffer 80 contain storage
for all associated per-vertex information, specifically including
normal and color and/or texture map coordinate.
For maximum storage space efficiency, when a vertex is specified in
the data stream, per vertex normal and/or color information
preferably is directly bundled with the position information.
Preferably, such bundling is controlled by two state bits: bundle
normals with vertices (BNV), and bundle colors with vertices (BCV).
FIG. 4E depicts a command structure including bits, among others.
When a vertex is pushed into the mesh buffer, these bits control if
its bundled normal and/or color are pushed as well.
It should be noted that compression according to the present
invention is not limited to triangles, and that vectors and dots
may also be compressed. Lines, for example, are a subset of
triangles, in which replacement bits are MOVE and DRAW. An output
vertex is then a vertex that represents one end point of a line
whose other vertex is the most recently, previously omitted vertex.
For dots, the replacement bits are DRAW, and an output vertex is
the vertex.
When CPU 52 executes a mesh buffer reference command, this process
is reversed. That is, the two bits specify whether a normal and/or
color should be inherited, or read, from the mesh buffer storage
80, or obtained from the current normal or current color. Software
58 preferably includes explicit commands for setting these two
current values. An exception to this rule exists, however, when an
explicit "set current normal" command is followed by a mesh buffer
reference, with the BNV state bit active. In this situation, the
former overrides the mesh buffer normal, to allow compact
representation of hard edges in surface geometry. Analogous
semantics are also defined for colors, allowing compact
representation of hard edges in surface colors.
Two additional state bits control the interpretation of normals and
colors when the stream of vertices is converted into triangles. A
replicate normals over triangle (RNT) bit indicates that the normal
in the final vertex that completes a triangle should be replicated
over the entire triangle. A replicate colors over triangle (RCT)
bit is defined analogously, as shown in the command structure of
FIG. 4E.
Compression of image xyz positions will now be described. Use of
the 8-bit exponent associated with 32-bit IEEE floating-point
numbers allows positions to range in size from sub-atomic particles
to billions of light years. But for any given tessellated object,
the exponent is actually specified just once by a current modeling
matrix, and object geometry is effectively described within a given
modeling space using only a 24-bit fixed-point mantissa. In many
cases far fewer bits are needed for visual acceptance. Thus
applicant's geometry compression language supports variable
quantization of position data down to one bit.
At the other extreme, empirical visual tests as well as well as
consideration of semiconductor hardware implementation indicate
that no more than 16 bits of precision per component of position is
necessary for nearly all cases.
Assume, however, that the position and scale of local modeling
space per object are specified by full 32-bit or 64-bit
floating-point coordinates. Using sufficient numerical care,
multiple such modeling spaces may be combined together to form
seamless geometry coordinate systems with much greater than 16-bit
positional precision.
Most geometry is local. Thus, within a 16-bit (or less) modeling
space for each object, the difference (.DELTA.) between adjacent
vertices in the generalized mesh buffer stream is likely to be less
than 16 bits in significance. If desired, one may construct a
histogram representing bit length of neighboring position delta's
in a batch of geometry, and based upon this histogram assign a
variable length code to compactly represent the vertices. As will
be described, preferably customized Huffman coding is used to
encode for the positional delta's in the geometry compression.
Compression of red-blue-green-alpha ("RBGA") colors will now be
described. Color data are treated similarly to positions, but with
a smaller maximum accuracy. Thus, RGB.alpha. color data are first
quantized to 12-bit unsigned fraction components that are absolute
linear reflectivity values (in which 1.0 represents 100%
reflectivity). An additional parameter allows color data
effectively to be quantized to any amount less than 12 bits. By way
of example, colors may all be within a 5-5-5 RGB color space, as
shown in FIG. 4C. The optional a field is controlled by a color
.alpha. present ("CAP") state bit shown in FIG. 4E. On the final
rendered image individual pixel colors are still interpolated
between the quantized vertex colors, and also typically are subject
to lighting.
As a design decision, it was decided to use the same delta-coding
for color components as is used for positions. The area of color
data compression is where geometry compression and traditional
image compression confront the most similar problems. However, many
advanced image compression techniques were avoided for geometry
color compression because of the difference in focus.
For example, the JPEG image compression standard relies upon
assumptions about viewing of the decompressed data that cannot be
made for geometry compression. For example, in image compression,
it is known a priori that the pixels appear in a perfectly
rectangular array, and that when viewed, each pixel subtends a
narrow range of visual angles. By contrast, in geometry
compression, the relationship between the viewer and the rasterized
geometry is unpredictable.
In image compression, it is known that the spatial frequency of the
displayed pixels upon on the viewer's eyes is likely higher than
the color acuity of the human visual system. For this reason,
colors are commonly converted to YUV space so that the UV color
components can be represented at a lower spatial frequency than the
Y (intensity) component.
Usually digital bits representing sub-sampled UV components are
divided among two or more pixels. However, geometry compression
cannot take advantage of this because there is no fixed display
scale of the geometry relative to the viewer's eye. Further, given
that compressed triangle vertices are connected to four to eight or
more other vertices in the generalized triangle mesh, there is no
consistent way of sharing "half" the color information across
vertices.
Similar arguments apply for the more sophisticated transforms used
in traditional image compression, such as the discrete cosine
transform. These transforms assume a regular (rectangular) sampling
of pixel values, and require a large amount of random access during
decompression.
It is known in the art to use pseudo-color look-up tables, but such
tables would required a fixed maximum size, and would represent a
relatively expensive resource for real-time processing. While
pseudo-color indices could yield slightly higher compression ratios
for certain scenes, the RGB model is more generalized and
considerably less expensive to implement.
In the RGB model used in the present invention, RBG values are
represented as linear reflectance values. Theoretically, if all
effects of lighting could be known a priori, one or two
representation bits could be dropped if the RGB components had been
represented in a nonlinear, or perceptually linear space (sometime
referred to as gamma corrected space). In practice, lighting
effects tend not to be predictable, and on-the-fly conversion from
nonlinear light to linear light would require considerable hardware
resources.
The compression of surface normals will now be described.
Traditionally 96-bit normals (three 32-bit IEEE floating-point
numbers) were used in calculations to determine 8-bit color
intensities. Theoretically, 96 bits of information could be used to
represent 2.sup.96 different normals, spread evenly over the
surface of a unit sphere. The resultant extremely high accuracy
represents a normal projecting in any direction every 2.sup.-46
radians.
But for IEEE floating-point normalized normals, the exponent bits
are effectively unused. Given the constraint N.sub.x.sup.2
+N.sub.y.sup.2 +N.sub.z.sup.2 =1, at least one of N.sub.x, N.sub.y,
or N.sub.z must be in the 0.5 to 1.0 range. During rendering, this
normal will be transformed by a composite modeling orientation
matrix:
Assuming a typical implementation in which lighting is performed in
world coordinates, the view transform is not involved in the
processing of normals. If the normals have been pre-normalized,
then to avoid redundant re-normalization of the normals, the
composite modeling transformation matrix T is typically
pre-normalized to divide out any scale changes. Thus :
During normal transformation, floating-point arithmetic hardware
effectively truncates all additive arguments to the accuracy of the
largest component. The result is that for a normalized normal
undergoing transformation by a scale preserving modeling
orientation matrix, the numerical accuracy of the transformed
normal value is reduced to no more than 24-bit fixed-point accuracy
in all but a few special cases.
By comparison, even 24-bit normal components would still provide
higher angular accuracy than the repaired Hubble space telescope.
In practice, some systems utilize only 16-bit normal components are
used. In empirical tests with 16-bit normal components, applicant
determined that results from an angular density of 0.01 radians
between normals were not visually distinguishable from finer
representations. This was about 100,000 normals distributed over a
unit sphere. In rectilinear space, these normals still require high
representation accuracy and as a design choice 16-bit components
including one sign and one guard bit were decided upon. This still
requires 48 bits to represent a normal, but since only 100,000
specific normals are of interest, theoretically a single 17-bit
index could denote any of these normals.
The use of normals as indices, and the resultant advantages
provided will now be described. One method of converting an index
of a normal on the unit sphere back into a N.sub.x, N.sub.y,
N.sub.z value is with a table look-up, the table being loaded into
memory 70 perhaps. Although table size is potentially large, the
requisite size can be substantially reduced by taking advantage of
a 48-way symmetry present in the unit sphere.
More particularly, as shown by FIG. 3, the unit sphere is
symmetrical by sign bits in the eight quadrants by sign bits. By
allowing three of the normal representation bits to be the three
sign bits of the xyz components of a normal, it then is only
necessary to represent one eighth of the unit sphere.
As shown by FIG. 3, each octant of the unit sphere can be divided
into six identical components by folding about the planes x=y, x=z,
and y=z. The six possible sextants are encoded with another three
bits, which leaves only 1/48 of the sphere remains to be
represented.
Utilizing the above-noted symmetry reduces the look-up table size
by a factor of 8.times.6=48. Instead of storing 100,000 entries,
the look-up table need store only about 2,000 entries, a size small
enough to be an on-chip ROM look-up table, stored perhaps within
ROM 59 (see FIG. 1). Indexing into the look-up table requires 11
address bits, which when added to the previously described two
3-bit fields results in a 17-bit field to describe all three normal
components.
Representing a finite set of unit normals is equivalent to
positioning points on the surface of the unit sphere. Although no
perfectly equal angular density distribution exists for large
numbers of points, many near-optimal distributions exist.
Theoretically, a distribution having the above-described type of
48-way symmetry could be used for the decompression look-up table
associated with the three-dimensional geometry decompression unit
130 (see FIG. 1).
However, several additional constraints mandate a different choice
of encoding. First, a scalable density distribution is desired,
e.g., a distribution in which setting in the look-up table more low
order address bits to "0" still results in fairly even normal
density on the unit sphere. Otherwise a different look-up table for
every encoding density would be required. Secondly, a
.DELTA.-encodable distribution is desired in that adjacent vertices
in geometry statistically have normals that are nearby on the
surface of the unit sphere. Nearby locations on the two-dimensional
space of the unit-sphere surface are most succinctly encoded by a
two-dimensional offset. It is desirable to have a distribution in
which such a metric exists. Finally, although computational costs
associated with the normal encoding process are not critically
important, distributions having lower encoding costs are still
preferred.
For these reasons the present invention utilizes a distribution
having a regular grid in the angular space within one sextant of
the unit sphere. As such, rather than a monolithic 11-bit index,
all normals within a sextant are advantageously represented with
two 6-bit orthogonal angular addresses. This configuration then
revises the previous bit-total to 18-bits. As was the case for
positions and colors, if more quantization of normals is
acceptable, these 6-bit indices can be reduced to fewer bits, and
thus absolute normals can be represented using anywhere from 18 to
as few as 6 bits. However, as described below, this space
preferably is .DELTA.-encoded to further reducing the number of
bits required for high quality representation of normals.
Normal encoding parameterization will now be described. Points on a
unit radius sphere are parameterized using spherical coordinates by
angles .theta. and .phi., where .theta. is the angle about the y
axis and .phi. is the longitudinal angle from the y=0 plane.
Equation (1) governs mapping between rectangular and spherical
coordinates as follows:
Points on the sphere are folded first by octant, and then by sort
order of xyz into one of six sextants. All table encoding takes
place in the positive octant in the region bounded by the half
spaces:
As shown in FIG. 3, the described triangular-shaped patch runs from
0 to .pi./4 radians in .theta., and from 0 to a maximum 0.615479709
radians in .phi..
Quantized angles are represented by two n-bit integers
.theta..sub.n and .phi..sub.n, where n is in the range of 0 to 6.
For a given n, the relationship between indices .theta. and .phi.
is: ##EQU1##
Equations (2) show how values of .theta..sub.n and .phi..sub.n can
be converted to spherical coordinates .theta. and .phi., which in
turn can be converted to rectilinear normal coordinate components
via equation (1).
To reverse the process, e.g. to encode a given normal N into
.theta..sub.n and .phi..sub.n, one cannot simply invert equation
(2). Instead, the N must be first folded into the canonical octant
and sextant, resulting in N'. Then N' must be dotted with all
quantized normals in the sextant. For a fixed n, the values of
.theta..sub.n and .phi..sub.n that result in the largest (nearest
unity) dot product define the proper encoding of N. Other, more
efficient methods for finding the correct values of .theta..sub.n
and .phi..sub.n exist, for example indexing through the table to
set .theta., and then jumping into .theta..
At this juncture, the complete bit format of absolute normals can
be given. The uppermost three bits specify the octant, the next
three bits the sextant, and finally two n-bit fields specify
.theta..sub.n and .phi..sub.n. The 3-bit sextant field takes on one
of six values, the binary codes for which are shown in FIG. 3.
Some further details are in order. The three normals at the corners
of the canonical patch are multiply represented, namely 6, 8, and
12 times. By employing the two unused values of the sextant field,
these normals can be uniquely encoded as 26 special normals.
This representation of normals is amenable to .DELTA.-encoding, at
least within a sextant, although with some additional work, this
can be extended to sextants that share a common edge. The .DELTA.
code between two normals is simply the difference in .theta..sub.n
and .phi..sub.n, namely .DELTA..theta..sub.n and
.DELTA..phi..sub.n.
Applicant's use of compression tags will now be described. Many
techniques are known for minimally representing variable-length bit
fields but for the geometry compression according to the present
invention, a variation of a conventional Huffman algorithm is
used.
The Huffman compression algorithm takes in a set of symbols to be
represented, along with frequency of occurrence statistics (e.g.,
histograms) of those symbols. From this, variable length, uniquely
identifiable bit patterns are generated that allow these symbols to
be represented with a near-minimum total number of bits, assuming
that symbols do occur at the frequencies specified.
Many compression techniques, including JPEG, create unique symbols
as tags to indicate the length of a variable-length data-field that
follows. This data field is typically a specific-length delta
value. Thus, the final binary stream consists of (self-describing
length) variable length tag symbols, each immediately followed by a
data field whose length is associated with that unique tag
symbol.
In the present invention, the binary format for geometry
compression uses this technique to represent position, normal, and
color data fields. For geometry compression, these <tag,
data> fields are immediately preceded by a more conventional
computer instruction set op-code field. These fields, along with
potential additional operand bits, will be referred to as geometry
instructions (see FIGS. 4A-4K).
Traditionally, each to be compressed value is assigned its own
associated label, e.g. an xyz .DELTA. position would be represented
by three tag-value pairs. But since the .DELTA.xyz values are not
uncorrelated, a denser, simpler representation can be attained. In
general, the xyz .DELTA.'s statistically point equally in all
directions in space. Thus, if n is the number of bits needed to
represent the largest of the .DELTA.'s, then statistically the
other two .DELTA. values require an average of n-1.4 bits for their
representation. The preferred embodiment therefore uses a single
field-length tag to indicate the bit length of .DELTA.x, .DELTA.y,
and .DELTA.z. although other design choices could have been
made.
Unfortunately, using this approach prevents taking advantage of
another Huffman technique to save somewhat less than one more bit
per component. However, the implemented embodiment outweighs this
disadvantage by not having to specify two additional tag fields
(for .DELTA.y and .DELTA.z). A further advantage is that using a
single tag field permits a hardware decompression engine to
decompress all three fields in parallel, if desired.
Similar arguments hold for .DELTA.'s of RGB.DELTA. values, and
accordingly a single field-length tag is used to indicate
bit-length of the R, G, B and, if present, .alpha., fields.
Absolute and .DELTA. normals are also parameterized by a single
value (n) that can be specified by a single tag. To facilitate
high-speed, low-cost hardware implementations, the length of the
Huffman tag field was limited to six bits, a relatively small
value. A 64-entry tag look-up table allows decoding of tags in one
clock cycle. One table exists for positions, another table exists
for normals, and yet another table exists for colors (and
optionally, also for texture coordinates). Each table contains the
length of the tag field, the length of the data field(s), a data
normalization coefficient, and an absolute/relative bit.
For reasonable hardware implementation, an additional complication
must be addressed. As described below, all instruction are
broken-up into an eight-bit header, and a variable length body,
sufficient information being present in the header to determine the
body length. But the header of one instruction must be placed in
the data stream before the body of the previous instruction to give
the hardware time to process the header information. For example,
the sequence . . . B0 H1B1 H2B2 H3 . . . has to be encoded as . . .
H1 B0 H2 B1 H3 B2 . . . .
The geometry compression instruction set used in the preferred
embodiment will now be described with respect to FIGS. 4A-4K. FIG.
4A depicts a vertex command that specifies a Huffman compressed
.DELTA.-encoded position, as well as possibly a normal and/or
color, depending on bundling bits (BNV and BCV). Two additional
bits specify a vertex replacement code (REP), and another bit
controls mesh buffer pushing of this vertex (MBP).
As shown in FIG. 4B, a normal command specifies a new current
normal and the color command shown in FIG. 4C depicts a new current
color. The normal command and color command each use Huffman
encoding of .DELTA. values.
The mesh buffer reference command structure is shown in FIG. 4D.
The mesh buffer reference command allows any of the sixteen most
recently pushed vertices (and associated normals and/or colors) to
be referenced as the next vertex. As further shown in FIG. 4D, A
2-bit vertex replacement ("REP") code is also specified.
FIG. 4E depicts the set state instruction that updates the five
state bits: RNT, RCT, BNV, BCV, and CAP.
FIG. 4F depicts a set table command, which is used to set entries
to the entry value specified in one of the three Huffman decoding
tables (Position, Normal, or Color).
FIG. 4G depicts a passthrough command that allows additional
graphics state not controlled directly by geometry compression to
be updated in-line.
FIG. 4H depicts a variable length no-op ("VNOP") command that
allows fields within the bit stream to be aligned to 32-bit word
boundaries. This permits aligned fields to be efficiently patched
at run-time by the general CPU 52.
FIGS. 4I, 4J and 4K respectively depict tag and .DELTA.-position
data structure, tag and .DELTA.-normal data structure, and tag and
.DELTA.-color data structure.
Those skilled in the art will recognize that instruction sets other
than what has been described above may instead be used to implement
the present invention.
The ratio of the time required for compression relative to
decompression is an important measure for many forms of
compression. In practice, it is acceptable for off-line image
compression to take up to perhaps sixty-times more time than
decompression, but for real-time video conferencing, the ratio
should be one.
Advantageously, geometry compression does not have this real-time
requirement. Even if geometry is constructed on the fly, most
geometry creating techniques, e.g., CSG, require orders of
magnitude more time than needed for displaying geometry. Also,
unlike continuous images found in movies, in most applications of
geometry compression a compressed three-dimensional object will be
displayed for many sequential frames before being discarded. Should
the three-dimensional object require animating, animation is
typically done with modeling matrices. Indeed for a CD-based game,
it is quite likely that an object will be decompressed billions of
times by customer-users, but will have been compressed only once by
the authoring company.
Like some other compression systems, geometry compression
algorithms can have a compression-time vs. compression-ratio
trade-off. For a given quality target level, as allowable time for
compression increases, the compression ratio achieved by a geometry
compression system increases. There exists a corresponding "knob"
for quality of the resulting compressed three-dimensional object,
and lower the quality knob, the better the compression ratio
achieved.
Aesthetic and subjective judgment may be applied to geometry
compression. Some three-dimensional objects will begin to appear
bad when target quantization of normals and/or positions is
slightly reduced, whereas other objects may be visually unchanged
even with a large amount of quantization. Compression can sometimes
cause visible artifacts, but in other cases may only make the
object look different, not necessarily lower in quality. In one
experiment by applicant, an image of an elephant actually begin to
appear more realistic, with more wrinkle-like skin, as the image
normals were quantized more. Once a model has been created and
compressed, it can be put into a library, to be used as
three-dimensional clip-art at the system level.
While many aspects of geometry compression are universal, the
above-described geometry compression instruction set has been
somewhat tailored to permit low-cost, high-speed hardware
implementations. (It is understood that a geometry compression
format designed purely for software decompression would be somewhat
different.). The preferred geometry compression instruction set is
especially amenable to hardware implementation because of the
one-pass sequential processing, limited local storage requirements,
tag look-up (as opposed to a conventional Hamming bit-sequential
processing), and use of shifts, adds, and look-ups to accomplish
most arithmetic steps.
FIG. 5 is a flowchart outlining method steps in a geometry
compression algorithm routine, according to the present invention.
Such routine may be stored in memory 80 and executed under control
of CPU 60 (see FIG. 1).
At step 200, an object is represented by an explicit group of
triangles to be compressed, along with quantization thresholds for
positions, normals, and colors. At step 210, a topological analysis
of connectivity is made, and hard edges are marked in normals
and/or color, if such information is not already present.
At step 220, vertex traversal order and mesh buffer references are
created, and at step 230 histograms of .DELTA.-positions,
.DELTA.-normals, and .DELTA.-colors is created. At step 240,
separate variable length Huffman tag codes are assigned for the
.DELTA.-positions, .DELTA.-normals, and .DELTA.-colors, based upon
histographs.
At step 250, a binary output stream is generated by first
outputting Huffman table initialization, after which the vertices
are traversed in order. Appropriate tags and .DELTA.'s are output
for all values.
Applicant has implemented a Wavefront OBJ format compressor that
supports compression of positions and normals, and creates full
generalized triangle strips, but does not yet implement a full
meshifying algorithm. Future embodiments will explore variable
precision geometry, including fine structured updates of the
compression tables. The present compressor expends time calculating
geometric details already known to the tessellator, and ultimately
it is hoped to generate compressed geometry directly. However, even
its present unoptimized state, applicant's software can compress
about 3,000 triangles/second in many cases.
At the user end, it is of course desirable to decompress the
compressed data, and the above-referenced patent application
describes a preferred manner of such decompression. An applicable
geometry decompression algorithm is set forth in ATTACHMENT 2, and
may be outlined as follows:
(1) Fetch the rest of the next instruction, and the first 8 bits of
the following instruction;
(2) Using the tag table, expand any compressed value fields to full
precision;
(3A) If values are relative, add to current value; otherwise
replace;
(3B) If mesh buffer reference, access old values;
(3C) If other command, do housekeeping.
(4) If normal, pass index through ROM table to obtain full
values.
(5) Output values in generalized triangle strip form to next
stage.
Applicant has implemented a software decompressor that successfully
decompresses compressed geometry at a rate of about 10,000
triangles/second. Hardware designs are in progress, a simplified
block diagram can be seen in FIG. 6. The rate of hardware
decompression may in the range of tens of millions of
triangles/second.
Comparative uncompressed and compressed image results are shown in
FIGS. 7A-7 and in Table 1, below. FIGS. 7A-7G depicts the same base
object, a triceratops, but with different quantization thresholds
on positions and normals. FIG. 7A is the original full
floating-point representation, using 96-bit positions and 96-bit
normals, denoted by the nomenclature P96/N96. FIG. 7B and 7C
depicts the effects of purely positional quantization P36/N96 and
P24/N96, respectively, while FIGS. 7D and 7E depict only normal
quantization, P96/N18 and P96/N12. FIGS. 5f and 5g show combined
quantization, P48/N18 and P30/N36.
FIGS. 7H-7L depict only quantized results for different objects,
including a galleon (P30/N12), a Dodge Viper (P36/N14), two views
of a '57 Chevy (P33/N13), and an insect (P39/N15).
Without zooming into the object, positional quantization much above
24-bits has essentially no significant visible effect. As the
normal quantization is reduced, the positions of specular
highlights on the surfaces are offset slightly. However, it is not
visually apparent that such changes are reductions in quality, at
least above 12 bits per normal. The quantization parameters were
photographed with the objects, and otherwise even applicant could
not distinguish between the original and most compressed versions
of the same object.
Table 1 summarizes compression and other statistics for these
objects. Column 1 notes the object in question, column 2 represents
the number of .DELTA.'s, and column three the .DELTA.-strip length.
The fourth column represents system overhead per vertex (overhead
being everything beyond position tag/data, and normal tag/data).
The "xyz quant" column denotes quantization thresholds, and the
sixth column depicts the number of bits/xyz. "Bits/tri" ninth
column depicts bits per triangle.
The results in Table 1 are measured actual compression data except
for estimated mesh buffer results, which are shown in parenthesis.
No actual mesh buffer results were present in that applicant's
prototype software compressor did not yet implement a full
meshifying algorithm. The estimate (in parenthesis) assumes a 46%
hit ratio in the mesh buffer.
In Table 1, the right-most column shows compression ratio
performance achieved over existing executable geometry formats.
Although total byte count of the compressed geometry is an
unambiguous number, in stating a compression ratio some assumptions
must be made about the uncompressed executable representation of
the object. Applicant assumed optimized generalized triangle
strips, with both positions and normals represented by
floating-point values to calculate "original size" data for Table
1.
To demonstrate the effect of pure 16-bit fixed point simple strip
representation, Table 1 also shows byte count for the mode of
OpenGL. As shown, average strip length decreased in the range of
2-3. Few if any commercial products take advantage of generalized
triangle strips, and thus Table 1 considerably understates
potential memory space savings.
TABLE 1
__________________________________________________________________________
Obj. .DELTA.stp ovrhd/ xyz bits/ norm bits/ bits/ org'l size comp.
size comp. name #.DELTA.'s len. vertex quant xyz quant norm tri
(bytes) (bytes) ratio
__________________________________________________________________________
tricer- 6,039 15.9 7.5 48 30.8 18 16.8 55.9 179,704 42,190 4.3X
atops (35.0) (26,380) (6.9X) tricer- 6,039 15.9 7.5 30 17.8 12 11.0
36.0 179,704 27,159 6.7X atops (24.4) (18,388) (9.8X) galleon 5,577
12.1 7.5 30 21.9 12 10.8 41.0 169,064 28,536 6.0X (27.2) (18,907)
(9.0X) Viper 58,203 23.8 7.5 36 20.1 14 10.9 37.5 1,698,116 272,130
6.3X (25.0) (181,644) (9.4X) 57 31,762 12.9 7.5 33 17.3 13 10.9
35.8 958,160 141,830 6.8X Chevy (24.3) (96,281) (10.0X) insect
263,783 3.0 7.5 39 22.8 15 11.0 51.5 9,831,528 1,696,283 5.8X
(33.9) (1,115,534) (8.9X)
__________________________________________________________________________
While certainly statistical variation exists between objects with
respect to compression ratios, general trends are nonetheless
noted. When compressing using the highest quality setting of the
quantization knobs (P48/N18), compression ratios are typically
about six. As ratios approach nearly then, most objects begin to
show visible quantization artifacts.
In summation, geometry compression according to the present
invention can represent three-dimensional triangle data with a
factor of six to ten times fewer bits than required with
conventional techniques. Applicant's geometry compression algorithm
may be implemented in real-time hardware, or in software. For a
fixed number of triangles, compression can minimize the total
bit-size of the representation, subject to quality and
implementation trade-offs. The resultant geometry-compressed image
suffers only slight losses in object quality, and may be
decompressed using software or hardware implementations. If
three-dimensional rendering hardware contains a geometry
decompression unit, application geometry may be stored in memory in
compressed format. Further, data transmission may use the
compressed format, thus improving effective bandwidth for a
graphics accelerator system, including shared virtual reality
display environments. The resultant compression can substantially
increase the amount of geometry cacheable in main memory.
To promote a fuller understanding of the role of the present
invention, especially in a compression-decompression system,
decompression of data that have been compressed according to the
present invention will now be described in detail. The following
description of decompression is taken from applicant's
earlier-referenced patent application.
FIG. 8 is a detailed block diagram of the decompressor unit 130,
shown in FIG. 1. As shown in FIG. 8, unit 130 includes a
decompression input first-in-first-out register ("FIFO") 200 whose
inputs include control signals and a preferably 32-bit or 64-bit
data stream, which signals and data stream preferably come from an
accelerator port data FIFO ("APDF") in interface unit 120 (see FIG.
1). The APDF portion of interface 120 includes a controller that
signals the size of the incoming data stream to unit 130. FIFO 200
provides output to an input block state machine 220 and to an input
block 210, state machine 220 and input block unit 210 communicating
with each other.
Output from block 210 is coupled to a barrel shifter unit 240 and
to a Huffman table set 230, the output from the Huffman look-up
being coupled to state machine 220. Opcode within state machine 220
processes the values provided by the Huffman tables 230 and outputs
data to the barrel shifter unit 240. State machine 220 also
provides an output to data path controller 260, which outputs a
preferably 12-bit wide signal to a tag decoder unit 294 and also
outputs data to the barrel shifter unit 240 and to a normal
processor 270, and a position/color processor 280.
Barrel shifter unit 240 outputs to the normal processor 270 and to
a position/color processor 280. The outputs from processors 270 and
280 are multiplexed by output multiplexer unit 290 into a
preferably 48-bit wide signal that is provided to a format
converter 292.
Decompression unit 130 generates a preferably 12-bit tag that is
sent to tag decoder 294 in parallel with either 32-bits or 48-bits
(for normals), that are sent to the format converter 292. These
data streams provide instructions that generate output to format
converter 292. A preferably 32-bit read-back path is used to
read-back the state of the unit.
Table 2, below, shows interface signals used to implement a
preferred embodiment of a decompression unit 130:
TABLE 2 ______________________________________ Signal Name Signals
I/O Description ______________________________________ id.sub.--
data 64 I Data inputs from APDF id.sub.-- tag 12 I Data on inputs
is valid from APDF fd.sub.-- stall 1 I Stall signal from format
converter di.sub.-- busy 1 O Busy signal to status register
di.sub.-- faf 1 O Fifo-almost- full signal-to- input FIFO df.sub.--
data 48 O Data output to formal converter df.sub.-- tag 12 O Tag
output to tag decoder du.sub.-- context 32 O Context output to UPA
section ______________________________________
Table 3, below, shows output data formats provided by unit 130. As
described herein, vertex, mesh buffer reference, and passthrough
instructions generate transactions from decompression unit 130.
Vertex and mesh buffer reference instructions send data to the
format converter, and each generates a header indicating vertex
replacement policy for the current vertex, followed by component
data. Each of these instructions always generates position data
and, depending upon the value of the state register, may contain
color or normal data. All three of the normal components preferably
are sent in parallel, whereas each position and color component is
separately sent. A passthrough instruction sends preferably 32-bits
of data to the collection buffer.
TABLE 3 ______________________________________ COMPONENTS FORMAT
______________________________________ Header 32. Position s.15
Color s.15 Normal s1.14(x3) Passthrough 32.
______________________________________
FIG. 9 is a detailed block diagram of the input block 210 depicted
in FIG. 8. A preferably 64-bit input register 300 receives data
from the APDF portion of interface 130, with 32-bits or 64-bits at
a time being loaded into register 300. Register 300 outputs
preferably 32-bits at a time via multiplexer 310 to a first barrel
shifter 320 whose output passes through a register 330 into a merge
unit 340. The 64-bit output from merge unit 340 is input to data
register 350, part of whose output is returned as input to a second
barrel shifter 360. The output from second barrel shifter 360 is
passed through a register 370 and is also input to merge unit 340.
First barrel shifter 320 aligns data to the tail of the bit-aligned
data stream being recycled from data register 350 through second
barrel shifter 360. The second barrel shifter 360 shifts-off the
used bits from data register 350.
FIG. 10 is a detailed block diagram of barrel shifter unit 240,
shown in FIG. 8. In overview, barrel shifter unit 240 expands the
variable-length position, color, and normal index components to
their fixed-point precisions. Data into unit 240 from unit 210
and/or 220 is input to a register 400 whose output is shown as
defining opcode and/or data units 410, 420, 430, 440, 450, and 460,
which are input to a multiplexer unit 470.
Multiplexer unit 470 input A is used for the X component of the
vertex instruction, input B is used for the set normal instruction
and the first component of the set color instructions, and input C
is used for the remaining components of the vertex and set color
instructions. Unit 240 further includes a barrel shift left
register 480 coupled to receive tag.sub.-- len data and to output
to register 490, whose output in turn is input to a barrel shift
right register 500 that is coupled to receive data.sub.-- len data.
Register 500 outputs to a mask unit 510 that is coupled to receive
shift data and whose output is coupled to register 520, which
outputs v.sub.-- data. The output of data block 460 is coupled to a
register 530 whose output is coupled to a second register 540,
which outputs pt.sub.-- data.
An appropriate table within Huffman tables 230 (see FIG. 8)
provides values of tag.sub.-- len, data.sub.-- len, and shift into
units 480, 500 and 510, respectively. Barrel shift left unit 480
shifts the input data left by 0 to 6 bits (tag.sub.-- len), thus
shifting off the Huffman tag. By contrast, barrel contrast, barrel
shift right register 500 shifts the data to the right by 0 to 16
bits (16.sub.-- data.sub.-- len), and sign extends the data, thus
bringing the data to its full size. Mask unit 510 masks off the
lower `shift` bits to clamp the data to the correct quantization
level.
FIG. 11 depicts in greater block diagram detail the position/color
processor unit 280, shown in FIG. 8. Processor unit 280 generates
final position or color component values. As shown in FIGS. 8 and
10, processor unit 280 receives a preferably 16-bit value (v.sub.--
data) from the barrel shifter unit 240, specifically mask unit 510
therein. If the abs.sub.-- rel bit from the Huffman table 230 is
set to relative, the incoming data are added by combiner unit 600
to the appropriate current stored data. The new value passes
through multiplexer 610, and is stored back into the register 620,
and is sent along to the output multiplexer 290, shown in FIG. 8.
However, if the abs.sub.-- rel bit is set to absolute, the incoming
data bypasses adder 600, is latched into the register 620, and is
also sent out to the output multiplexer 290. As shown in FIG. 11,
the position/color processor unit 280 further includes a
position/color mesh buffer 630 that is coupled to receive the input
to register 620. The output from mesh buffer 630 is coupled to
multiplexer gates, collectively 640, whose outputs reflect current
values of x, y, z, r, g, b and .alpha.. A register set,
collectively shown as 650, provides these current values to the
input of a multiplexer 660, whose output is coupled to the adder
600. Processor unit 280 further includes a register 670 that
receives and outputs pt.sub.-- data from barrel shifter unit
240.
As shown in FIG. 8, normal processor unit 270 also outputs data to
the output multiplexer 290. FIG. 12A depicts in detail the
sub-units comprising normal processor unit 270. As seen in FIG. 8
and FIG. 10, the normal processor unit 270 receives an 18-bit
normal index as three separate components: sextant/octant, u and v,
or encoded .DELTA.u and .DELTA.v components from mask unit 510 in
barrel shifter unit 240. If the value is a .DELTA.-value
(relative), the .DELTA.u and .DELTA.v are added to the current u
and v values by respective adders 710. The intermediate values are
stored and are also passed on to a fold unit 800 associated with
decoder-fold-rom unit 272 (see FIG. 12B).
As shown in FIG. 12A, the normal processor unit 270 further
includes registers 712, 714, 716, 718, 720, 722, 724, 726 which
hold respective octant, sextant, u and v values, curr.sub.-- oct,
curr.sub.-- sext, curr.sub.-- u and curr.sub.-- v values. Also
present in unit 270 are multiplexers 740, 742, 744, 746, 748, 750,
752, 754, 756, 758 and 760, 1's complementing units 770, 772,
latch-flipflop units 780, 782, 784 for holding respective v, u, and
uv information, further adders 790, 792, and a normal mesh buffer
794 coupled to receive curr.sub.-- normal input components.
With reference to FIGS. 12A and 12B, for an absolute reference, the
u and v values are passed directly to fold unit 800. The octant and
sextant portions of the index are sent to decoder 810, within unit
272. Decoder 810 controls multiplexer 820 (which select constants),
as well as multiplexers 840, 842, 844, 860, 862, 864, which reorder
components, and invert signs (using 2's complement units 850, 852,
854).
Fold unit 800 uses the u and v components of the normal index, from
unit 270, to calculate the address into the normal look-up table
ROM 830. The octant and sextant fields, from unit 270, drive a
decoder 810 that determines sign and ordering of components output
from the ROM look-up table 830. Decoder 810 also handles special
case normals not included in the normal ROM look-up table 830.
FIG. 13 depicts interfaces to a mesh buffer, as shown in FIG. 11
and/or FIG. 12A. Preferably, mesh buffer 794 is implemented as a
register file and a pointer to the current location. Data is input
to the mesh buffer FIFO at the position of the current location
pointer. However, random access to any of the 16 locations is
allowed when reading the data out of the FIFO by indexing off this
pointer: address=(curr.sub.-- loc.sub.-- ptr--index) mod 16.
FIG. 14A depicts interfaces to Huffman tables, e.g., tables 230 in
FIG. 8. Huffman tables are used to decode the Huffman tags
preceding the compressed data. Three Huffman tables are used: one
for position, for color, and for normal data, with each table
preferably holding 64 entries.
FIG. 14B depicts a preferred format for entry of position, v and
normal color data in the Huffman tables. The instruction format for
loading the Huffman tables in the compressed data stream is
described later herein.
Several instructions generate data for the format converter 292,
shown in FIG. 8, and appropriate tags must be generated for this
data so the format converter can correctly process the data. Table
4, below, shows tags generated for the different data components.
The components that show two tags may set the launch bit, and the
second tag shows the value with the launch bit set.
TABLE 4 ______________________________________ COMPONENTS TAG
______________________________________ Header 0x020 X 0x011 Y 0x012
Z 0x013/0x413 Nx/Ny/Nz 0x018/0x418 R 0x014 G 0x015 B 0x016/0x416 A
0x017/0x417 U 0x0c0/0x4c0 V 0x01c/0x41c
______________________________________
Input block state machine 220 (see FIG. 8) includes a preferably
six-bit state register that holds information about the processing
state of the decompression unit. Preferably, the following state
bits are defined:
Bit 5: tex--Texture values in place of color
Bit 4: rnt--Replicate normal per vertex
Bit 3: rct--Replicate color per vertex
Bit 2: bnv--Normal bundled with vertex
Bit 1: bcv--Color bundled with vertex
Bit 0: cap--Color includes alpha (.alpha.)
Position/Color processor unit 280 (see FIGS. 8 and 11) preferably
includes three 16-bit registers, curr.sub.-- x, curr.sub.-- y, and
curr.sub.-- z, which contain the current position components, X, Y,
and Z, and are only updated by vertex instructions.
Normal processor unit 270 (see FIGS. 8 and 12A) preferably includes
three six-bit registers, curr.sub.-- oct, curr.sub.-- sext,
curr.sub.-- u, curr.sub.-- v) that contain the current normal. The
first register holds the 3-bit sextant and octant fields, and the
remaining two registers contain the u and v coordinates for the
normal. These values are written using the set normal instruction,
or they are updated by the vertex instruction if the bnv bit is set
in the state register.
Position/color processor 280 further preferably includes four
16-bit registers, curr.sub.-- r, curr.sub.-- g, curr.sub.-- b,
curr.sub.-- a, which contain the current color components, red,
green, blue and alpha (.alpha.). These components are set using the
se5t color instruction, or they are updated by the vertex
instruction if the bcv bit is set in the state register.
Preferably, alpha is valid only if the cap bit is set in the state
register. The test bit is set when processing texture components,
in which case only red and green are valid.
A preferred instruction set implementing decompression of data
compressed according to the present invention will now be
described. FIG. 15A depicts the vertex instruction format, an
instruction that uses variable-length Huffman encoding to represent
a vertex. Position information is always present in this
instruction.
(REP) The vertex replacement policy is as follows:
00--Restart clockwise
01--Restart counter-clockwise
10--Replace middle
11--Replace oldest
(M)--mesh buffer push:
0--No push
1--Push
With reference to FIG. 15A, the position data consists of a
variable-length Huffman tag (0 to 6 bits) followed by three data
fields of equal length for the X, Y, and Z components, which are
either .DELTA.-values or absolute values. The data.sub.-- len field
for the entry in the position Huffman table gives the length of
each of the X, Y, and Z fields, the tag.sub.-- len entry gives the
length of the tag, and the abs.sub.-- rel entry tells whether the
data is absolute data or is relative to the previous vertex. The
shift entry from the Huffman table gives the quantization level
(number of trailing zeroes) for the data.
If the bnv bit is set in the state register, a normal is included.
The encoded normal has a Huffman tag followed by either two
variable-length data fields for .DELTA.u and .DELTA.v, or a
fixed-length field for the sextant and octant (6 bits) followed by
two variable-length fields for u and v. The former encoding is for
delta encodings of normals, while the latter encoding is for
absolute encodings. The data.sub.-- len, tag.sub.-- len, abs.sub.--
rel, and shift fields from the normal Huffman table are used
similarly as entries from the position table.
FIG. 15B depicts vertex component data formats. If the bcv bit in
the state register is set, color is included with the vertex. The
color is encoded similar the position, using three or four fields,
but how the fields are used is determined by the tag table. If
tagged absolute, then x, y, z, r, g, b data is used. Absolute
normals are used with sectant and octant fields. However, if the
tag table indicates relative, delta normals are used, and it
suffices to send latitude and longitude data (e.g., .theta. and
.PHI., also referred to herein as u and v.
With further reference to FIG. 15B, a Huffman tag is followed by
three equal length fields for R, G, and B. The cap bit in the state
register indicates whether an additional field for .alpha. is
included. The data.sub.-- len, tag.sub.-- len, abs.sub.-- rel, and
shift fields from the color Huffman table are used similarly as for
entries from the position and normal tables.
The states of the vertex instruction set are as follows:
1. Latch next opcode; output X; shift barrel shift right unit 500
(see FIG. 10) by ptag.sub.-- len+pdata.sub.-- len--pquant+2.
2. Merge; output Header.
3. Output Y; shift barrel shift right unit 500 (see FIG. 10) by
pdata.sub.-- len--pquant.
4. Merge
5. Output Z; shift barrel shift right unit 500 (see FIG. 10) by
pdata.sub.-- len--pquant.
6. Merge.
a. If (bnv)
i. if (absolute normal), goto 7,
ii. else goto 9. /*relative normal*/
b. else If (rnt), goto 21,
c. else If (bcv) goto 13,
d. else If (rct) goto 22,
e. else Merge; branch to next instruction.
7. Latch next opcode; output sextant/octant; shift barrel shift
right unit 500 (see FIG. 10) by ntag.sub.-- len+6.
8. Merge.
9. Output U.
a. If (absolute normal), shift barrel shift right unit 500 (see
FIG. 10) by ndata.sub.-- len--nquant.
b. else/*relative normal*/, latch next opcode; shift Bs2 by
ntag.sub.-- len+ndata.sub.-- len--nquant
10. Merge.
11. Output V.
12. Merge.
a. If (bcv), goto 13,
b. else If (rct), goto 22,
c. else Merge; branch to next instruction.
13. Latch next opcode; output R; shift barrel shift right unit 500
(see FIG. 10) by ctag.sub.-- len+cdata.sub.-- len--cquant.
14. Merge
15. Output G; shift barrel shift right unit 500 (see FIG. 10) by
cdata.sub.-- len--cquant.
16. Merge; if (tex), branch to next instruction.
17. Output B; shift barrel shift right unit 500 (see FIG. 10) by
cdata.sub.-- len--cquant.
18. Merge; if (cap) branch to next instruction.
19. Output A; shift barrel shift right unit 500 (see FIG. 10) by
cdata.sub.-- len--cquant.
20. Merge; branch to next instruction.
21. Output curr.sub.-- normal.
a. If (bcv), goto 13,
b. else If (rct), goto 22,
c. else Merge; branch to next instruction.
22. Output curr.sub.-- r.
23. Output curr.sub.-- g. If (tex), Merge; branch to next
instruction
24. Output curr.sub.-- b. If (cap), Merge; branch to next
instruction.
25. Output curr.sub.-- a. Merge branch to next instruction.
FIG. 15C depicts the format for the set normal instruction. The set
normal instruction sets the value of the current normal registers.
The normal data is encoded similarly as is normal data in the
vertex instruction, described herein. The states of the set normal
instruction are as follows:
If (absolute normal)
1. Latch next opcode; output sextant/octant; shift barrel shift
right unit 500 (see FIG. 10) by ntag.sub.-- len+8.
2. Merge.
3. Output U; shift barrel shift right unit 500 (see FIG. 10) by
ndata.sub.-- len--nquant.
4. Merge.
5. Output V; shift barrel shift right unit 500 (see FIG. 10) by
ndata.sub.-- len+nquant.
6. Merge; branch to next instruction. else/*relative normal*/
1. Latch next opcode; output dU; shift barrel shift right unit 500
(see FIG. 10) by n.sub.-- tag.sub.-- len+ndata.sub.--
len--nquant.
2. Merge.
3. Output dV; shift barrel shift right unit 500 (see FIG. 10) by
ndata.sub.-- len--nquant.
4. Merge; branch to next instruction.
FIG. 15D depicts the set color instruction, an instruction that
sets the value of the current color registers. Encoding of the
color data is similar to encoding of the color data in the vertex
instruction. The states of the set color instruction are as
follows:
1. Latch next opcode; output R; shift barrel shift right unit 500
(see FIG. 10) by ctag.sub.-- len+cdata.sub.-- len--cquant+2.
2. Merge.
3. Output G; shift barrel shift right unit 500 (see FIG. 10) by
cdata.sub.-- len--cquant.
4. Merge. If (tex), branch to next instruction.
5. Output B; shift barrel shift right unit 500 (see FIG. 10) by
cdata.sub.-- len--cquant.
6. Merge. If (cap) branch to next instruction.
7. Output A; shift barrel shift right unit 500 (see FIG. 10) by
cdata.sub.-- len--cquant.
8. Merge; branch to next instruction.
FIG. 15E is a preferred format for the mesh buffer reference
instruction. This instruction causes data from an entry in the mesh
buffer to be sent out to the format converter as the next vertex.
With reference to FIG. 15E, the index indicates the entry from the
mesh buffer to send. The newest entry in the mesh buffer has index
0, and the oldest has index 15. REP, the above-described
replacement policy for the vertex instruction, is the same as used
for the mesh buffer reference instruction. The states for the mesh
buffer reference instruction are as follows:
1. Latch next opcode; output Header; shift barrel shift right unit
500 (see FIG. 10) by 9.
2. Output X from mesh buffer.
3. Output Y from mesh buffer.
4. Output Z from mesh buffer.
a. If (bnv or rnt) goto 5,
b. else If (bcv or rct) goto 6,
c. else Merge; branch to next instruction.
5. If (bnv), output Normal from mesh buffer, else if (rnt) output
curr.sub.-- normal.
a. If (bnv or rct) goto 6,
b. else Merge; branch to next instruction.
6. If (bcv), output R from mesh buffer, else if (rct) output
curr.sub.-- r.
7. If (bcv), output G from mesh buffer, else if (rct) output
curr.sub.-- g. If (tex), Merge; branch to next instruction.
8. If (bcv), output B from mesh buffer, else if (rct) output
curr.sub.-- b. If ( cap), Merge; branch to next instruction.
9. If (bcv), output A from mesh buffer, else if (rct) output
curr.sub.-- a. Merge; branch to next instruction.
FIG. 15F depicts the set state instruction, which sets the bits the
decompression unit state register. The states for the set state
instruction are as follows:
1. Latch next opcode; shift barrel shifter 2 by 11 bits.
2. Merge; branch instruction
FIG. 15G depicts the set table instruction, which sets Huffman
table entries. The table selection is as follows:
00--Position table
01--Color table
10--Normal table
11--Undefined
The tag length is derived from the address. The nine bits in the
entry field correspond to the absolute/relative bit, data length,
and shift amount fields of the Huffman table entries. (The
preferred format of the Huffman table entries has been described
earlier herein.) The states of the set table instruction are as
follows:
1. Latch next opcode; send address and entry to Huffman tables;
shift barrel shift right unit 500 (see FIG. 10) by 23.
2. Merge; branch to next instruction. Table 5, below, shows the
preferred Huffman Table Fill Codes.
TABLE 5 ______________________________________ Entries Tag Address
Filled Length Fill Range ______________________________________
0tttttt 1 6 tttttt 10ttttt 2 5 ttttt0-ttttt1 110tttt 4 4
tttt00-tttt11 1110ttt 8 3 ttt000-ttt111 11110tt 16 2 tt0000-tt1111
111110t 32 1 t00000-t11111 1111110 64 0 Entire table
______________________________________
FIG. 15H depicts the passthrough instruction, which allows
passthrough data to be encoded in the compressed-data stream. The
length of the instruction preferably is 64-bits. Aligning
successive passthrough instructions to a 64-bit boundary allows for
patching of passthrough data in the encoded stream. The states for
the passthrough instruction are as follows:
1. Latch next opcode; read address, shift barrel shift right unit
500 (see FIG. 10) by 32 bits.
2. Merge.
3. Output data, shift barrel shift right unit 500 (see FIG. 10) by
32 bits.
4. Merge; branch to next instruction.
FIG. 15I depicts the variable-length NOP ("VNOP) instruction, which
encodes a variable number of 0 bits in the data stream. The
five-bit count shown in FIG. 15I designates the number of 0 bits
that follow. This instruction is implicitly used for the start of
the data stream. This instruction may also be used to pad the data
stream to 32-bit or 64-bit boundaries, or encoding regions, for
later patching. The states for this instruction are:
1. Latch next opcode; read count; barrel shift right unit 500 (see
FIG. 10) by 13 bits;
2. Merge.
3. Barrel shift right unit reads "count" positions;
4. Merge; branch to next instruction.
FIG. 15J shows the skip 8 instruction, whose states are:
1. Latch next opcode; shift barrel shift right unit 500 (see FIG.
10) by 16 bits;
2. Merge; branch to next instruction.
It will be appreciated that it may be advantageous to reduce
bandwidth requirements between devices by not decompressing a data
stream at a single point in a decompression system. It will be
appreciated that parallel decompression of a data stream may be
implemented by providing an additional command advising the arrival
of a given number of data words that may be processed in
parallel.
The presence of such parallel opportunities may be recognized by
the presence of mark bits, at which occurrence the stated number of
data words may be shuttled to other processors within the system
for parallel decompression. Further, it is then permissible to jump
ahead the given number of words in the data stream to arrive at the
next data that is not eligible for parallel processing.
Further, morphing capability may be implemented to eliminate any
abrupt perception gap in viewing a decompressed three-dimensional
object. Within the decompressed data stream, it is possible to
specify vertices as linear or other interpolations of vertices that
are actually present or have previously been decompressed. Assume,
for example, that the three-dimensional object is a tiger. At a far
distance, no teeth are present in the tiger's mouth, yet at near
distance teeth are present. The result is a seamless transition
such that as distance to the tiger shrinks, the teeth grow, with no
sudden change seen between a toothless tiger and a toothed
tiger.
Modifications and variations may be made to the disclosed
embodiments without departing from the subject and spirit of the
invention as defined by the following claims.
* * * * *